Electric machines may be equipped with cooling devices for cooling rotating electric coil windings. Machines comprising superconductive rotor windings, in particular, are typically equipped with cooling devices in which a coolant such as liquid nitrogen, liquid helium, or liquid neon circulates in the interior of a central rotor shaft according to the thermosiphon principle and can thereby remove heat from the rotor. With the aid of such cooling systems, superconducting coil windings, in particular superconducting rotating excitation windings, can be cooled to an operating temperature below the transition temperature of the superconductor and can be held at this operating temperature.
In the case of such known cooling devices, an end region of the rotor shaft is often used to feed coolant, which has been liquified by a fixed cooling unit, into an inner chamber of the rotor shaft, for example, via a fixed coolant tube which protrudes into the rotor shaft. From the inner chamber of the rotor shaft, the coolant can travel via separate radial lines to the radially further outward positions of the coil windings. Such a cooling device is known, for example, from EP 2 603 968 A1.
These known cooling devices are suitable for cooling electric machines that have only a small number of coil windings. In the case of high pole-count machines having a high number of coil windings, however, the use of a separate radial coolant line for each coil winding would result in a highly complex apparatus.
For this reason, an alternative cooling concept was proposed for high pole-count machines such as those used, for example, in wind-power or hydropower generators. Such an alternative cooling device is described in EP 2 521 252 A1. Therein, the cooling device comprises a plurality of cold heads rotating together with the rotor. The heat to be transported away from the coil windings is carried away via thermal conduction into solid copper elements which are thermally connected to the cold heads. As a result, the feeding of coolant into the rotor shaft and the distribution of the coolant into radially outer positions is completely avoided.
One disadvantage of this cooling concept, however, is that the sensitive and heavy cold heads must be designed for high rotational speeds. One further disadvantage is that, due to the solid copper elements for cooling the coils, a relatively high additional co-rotating mass is present.